That's real neat to see more people are getting into the idea.
I was going to run a dedicated vintage oriented email server on the domain vtools.68kmla.org but at least with AppleShare IP 6, there's no way to require or ask for authentication of any kind on sending, so it immediately got used as a spam relay and also filled the service's logs, so I had to turn off the service. I haven't yet made time and sat down with the manuals to OS X Server 10.4 to see what might be possible there.
Though, really what's probably a better idea is turning on insecure access on a more modern server, so that's a set of decisions for Future Cory, or someone else.
The other concern I have, and the main reason I wouldn't even suggest anything like trying to use your modern email account, is email volume is potentially hundreds of times what it was in the '90s. I'm not confident any mail software on Classic Mac OS could handle, say, my work or personal email, each of which get over 100 messages a day.
A G4 with OS X software and an SSL proxy or an appropriately security-lowered server might be able to handle that mail volume, but the question is kind of... "and then what?"
At work, very close to 100% of the email I get is either advertisements i don't want or an automated notification with a call to go click on a link and handle something in a ticket. At home, the bulk of the volume I get is notifications from web sites (e.g. you got a DM on twitter, someone mentioned you on the forum) or advertisements from shops where I have accounts or rewards cards.
I can see maybe making a mailing list account.
<joke about how much I want to run a listserv, even though most people I know wouldn't want to be on mailing lists, even a peer to peer discussion list>
For beige Macs, I don't really mind. "beige' and "internet" only really crossed over for a couple years for me (personally, I know it was longer for others) and the interaction model at the time was webmail and IRC.
We do have an IRC channel for the site, so that's an option. I know there's plenty of plain HTML and no-HTTPS options for things like news, proxying other web sites, etc etc, so there's some choices and this will all be a personal goals and style thought process.
I don't know much about it but I know Cameron Kaiser (the tenfourfox/classilla guy) was running a gopher system and maybe a gopher proxy? That's contemporary with most beige macs so may be worth looking at. There's also links to some BBSes that pop up in The Lounge from time to time!